A waste heat steam generator is a heat exchanger that recovers heat from a hot gas flow. Waste heat steam generators are often used in gas and steam turbine plants (combined cycle plants), which are used predominantly for power generation. A modern combined cycle plant usually comprises one to four gas turbines and at least one steam turbine, either each turbine respectively driving a generator (multi-shaft plant) or one gas turbine acting together with the steam turbine on a common shaft to drive a single generator (single-shaft plant). The hot waste gases of the gas turbine are in this case used in the waste heat steam generator for generating steam. The steam is subsequently fed to the steam turbine. Usually, about two thirds of the electrical power is accounted for by the gas turbine and one third is accounted for by the steam process.
By analogy with the various pressure stages of a steam turbine, the waste heat steam generator also comprises a plurality of pressure stages with different thermal states of the water-steam mixture respectively contained. In each of these pressure stages, on its flow path the flow medium passes firstly through economizers, which use residual heat for preheating the flow medium, and subsequently various stages of evaporator and superheater heating surfaces. In the evaporator, the flow medium is evaporated and the steam obtained is heated further in the superheater. In most present-day combined cycle plants, recirculating waste heat steam generators are used in all the pressure stages in which the waste heat steam generator has a central drum that is in connection with all three sections of the steam generator on the water side, the evaporator, the economizer and the superheater. The liquid phase always remains in the drum and is fed continuously to the evaporator via downcomers. Steam bubbles rise up to the surface of the water and are drawn off from the drum at the top. The steam produced is made up by replenishing feedwater from the economizer. Thus, the water level in the drum is kept almost constant.
In recirculating evaporators with a customary drum size, the feedwater flow is today mostly provided by what is known as a three-component control. The desired value for the feedwater mass flow is chosen in dependence on the live steam mass flow. The prime objective of this feedwater control is to maintain the desired water level in the drum. For this reason, the water level in the drum at any given time serves as a corrective controlled variable which, depending on the deviation from the desired value, brings about a corrective change in the feedwater mass flow. On account of the large store of water in the drum (buffer volume), which allows the corrective controller to operate slowly, critical transient processes, such as for example rapid changes in load, can also be handled within limits from the viewpoint of a water level in the drum that fluctuates little and is admissible.
However, modern power generating plants are required on the one hand to deliver high degrees of efficiency, which involves an increase in the steam parameters if only because of higher gas turbine outlet temperatures, and on the other hand to operate as flexibly as possible. Because of the high pressures and temperatures, drums in large boilers have very great wall thicknesses. When there is rapid heating up or cooling down, these great wall thicknesses in turn lead to thermal stresses, which may be as much as the load limit of the material. Thick-walled drums therefore restrict the maximum admissible running-up and running-down gradients of the steam generator. In order to limit the wall thicknesses to an acceptable amount, the drum diameter has to be reduced.
If therefore, on account of more demanding requirements with regard to the steam parameters and the flexibility of the plant, the drum diameter is reduced, the requirements for the feedwater control concept, and consequently also for the filling level control of steam drums in recirculating evaporators, increase. Under some circumstances it may be the case that a slow-acting three-component control that still produces reasonable results with a large buffer volume can no longer be meaningfully used here.